Topic 12: Magnetic resonance imaging (MRI)
Physics Objectives category 1 • Describe the Nuclear Magnetic Resonance (NMR) phenomenon from both classical physics and quantum mechanics perspectives. • Discuss the significance and the uniqueness of the Larmor frequency for a nuclear species. • Describe the origin of the Free Induction Decay and discuss the key factors which determine its strength. • Describe the origin of the T1 and T2 relaxation mechanisms. • Describe the behaviour of T1 and T2 as the strength of the static field is changed. • Describe the spin-echo and inversion recovery pulse sequences – including multiple spin echo and STIR. • Outline the advantages and characteristic features of Gradient Echo, Fast Spin Echo, Echo Planar Imaging (EPI) and other fast imaging techniques. • Discuss the physics behind the chemical shift phenomenon. • Describe how gradients may be applied to spatially encode the NMR signal. • Describe interleaved multislice imaging and indicate why it is utilised. • Discuss quality features of MR images including artifacts. • Discuss safety issues (patient and environmental) and contra-indications in the use of MRI. Topics category 1 (a) Basic Nuclear Magnetic Resonance (NMR): • Magnetic susceptibility • Nuclear magnetic moments • Effect of external magnetic field • Nuclear precession • Equilibrium magnetization • Significance of Radio Frequency (RF) pulse • Resonance & Larmor frequency • Free Induction Decay (FID) • Chemical shift (b) Relaxation: • longitudinal (T1) relaxation time • Transverse (T2) relaxation time • Effect of field inhomogeneities, T2* © Pulse sequences: • Inversion recovery and STIR • Spin-echo • Characteristic features of Gradient Echo, Fast Spin Echo, Echo Planar Imaging (EPI) and other fast imaging techniques (d) Production of the Image: • Gradient fields • Slice thickness and RF bandwidth • Phase-encoding gradient • Frequency-encoding (readout) gradient • Determinants of image acquisition time (e) Image Quality • Signal-to-noise ratios • Spatial resolution • Common artefacts (f) Hazards and bio-effects: • Static magnetic field • Time varying magnetic field • RF field • Contraindications for MRI • Environmental problems Objectives category 2 • Discuss the role of the Fourier Transform (FT) in MR image reconstruction. • Describe 2D-FT reconstruction methods in terms of the three time intervals (slice selection, phase encoding and frequency encoding). • Compare the 3D-FT reconstruction technique with the 2D-FT method. • Discuss the advantages of the Gradient Echo, Fast Spin Echo, Echo Planar Imaging (EPI) and other fast imaging techniques. • Explain the effects of preparatory inversion pulse on image contrast. • Compare and contrast fat suppression obtained by spectral, IR GRE and subtraction methods. • Identify the biomolecular species which may be analysed in clinical MRS. Topics category 2 (a) Production of the Image: • 2D Fourier transformation technique • 3D Fourier transformation technique (b) Fat suppression and fat imaging © Magnetic resonance spectroscopy (MRS) (d) Contrast imaging Objectives category 3 • Describe the general construction and mode of operation of MRI scanners. • Describe in simple terms the effects of blood flow on MR image data. Topics category 3 (a) Instrumentation • Magnets • Gradient coils • RF coils and electronics • Functional MRI (b) Hybrid MR-PET © Intra operative (d) Flow effects: • Flow-void effect • Paradoxical enhancement • Magnetic Resonance Angiography (MRA) • Diffusion imaging